We typically induction heat the upset ends of long drill pipe for the oil industry. Our forging process takes place at 2100-2300ºF. The steel we use is typically 4140/4330. At what temperature would incipient melting occur, and at what temperature would we start seeing a drooping effect? We are concerned that the ends stay concentric to the body. Also, when normalizing, should all the residual stresses caused from forging or cold working be taken out? We normalize and temper in a continuous furnace after forging.
The forging temperature for 4140/4340 material – whether solid bar or pipe – should be 2250ºF or below. The higher you go above this temperature, the weaker the material becomes and the more drooping you will experience. It is really an exponential relationship. A small increase in temperature has a profound effect on the strength of the material, especially above about 1800ºF.
There may be several reasons for incipient melting. First, if your process is out of control and you are (locally) approaching the melting point of the material (typically around 2800ºF), melting may occur. Also, if you have any alloy segregation or a non-homogeneous microstructure, incipient melting can occur. In most cases, however, it is an indication of improper induction parameters (see below) or controls/sensors reading incorrectly.
Obviously, the hotter the part, the easier it is on the die set (i.e. longer die life), so there is always a compromise. If you do reach 2300ºF, you may want to do so only at the end of the bar, not throughout the heated length. A quick “rule of thumb” is to add one diameter to the forged length to determine the length you should be heating (e.g., a 1.5"-diameter tube with a 7" forged length requires an 8.5” <7” + 1.5”> heated length). You really don't want to heat more of the bar than you need to. Again, this will contribute to greater distortion. By an innovative coil design, you can even make the end hotter than the rest of the bar if required.
Also, you might be at the wrong frequency, causing longer soak times and increasing the risk of drooping. Be sure you have taken into account the range of sizes of pipe you are running (diameters and thickness of material in the area of interest) and adjust the induction parameters being used - power (kW) and frequency (Hz) - accordingly. Tempil® paint or other temperature measurement tools will help you determine the temperatures being achieved during setup.
Finally, a "good" normalize will remove all residual stresses caused by forging and/or cold working to the point where the microstructure of the material shows no evidence of flow lines, etc. A minimum of one hour per inch or cross sectional area but no less than two hours at temperature is recommended. This is often difficult to achieve in continuous furnaces due to production constraints. Hence, more compromises may be necessary.